Avian bird flu is virus that is found naturally within the digestive systems of wild birds, who generally remain asymptomatic. However, these flu strains are often highly contagious and can spread to domesticated birds, causing sickness and death. The most pathogenic strains can cause entire flocks of birds to become severely ill, often resulting in death within just 48 hours of infection.

Some subtypes of avian influenza can be transmitted to people, and the H5N1 virus has been identified as a highly pathogenic strain that has the potential to become a pandemic threat. H5N1 has killed millions of poultry worldwide and 252 humans as of January 2009, according to the WHO. At this point, it is very rare for a person to become infected with the H5N1 virus and generally only occurs among those who have very close contact with infected birds. Person-to-person transmission has only ever occurred between blood relatives and is also extremely unlikely. However, it is feared that the H5N1 virus could mutate into a strain that is more harmful and contagious among humans.

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